Most commercially available automotive engines used fixed lift and duration intake and exhaust valve events. As a result, there must be a compromise between the best fuel economy, emission control, and engine power conditions.
Potentially better fuel economy, emission control and engine output benefits can be realized if the timing of these events can be varied depending on the engine operating modes. Further enhancement also can be obtained by the use of reed or one-way valves in the intake port. These would control the flow of intake and exhaust gases as a function of engine cylinder pressure, while at times providing internal exhaust gas recirculation (EGR) when needed to control oxides of nitrogen and unburned hydrocarbons (HC) levels . Further, utilizing unthrottled operation as much as possible would eliminate/minimize the engine pumping losses.